@rpapo Well, some comes from the home no doubt, but school kids have their own social dynamic going. It is possible for schools to guide that dynamic away from bullying if they start early, but most have no idea how and end up instead miserably failing to stop it after it's well and truly taken hold.
Personally, I was a shrimp, bookish, quiet and glasses-wearing. I was bullied relentlessly in elementary school, frequently chased around the schoolyard by little gangs and beaten up if caught. And of course tons of taunting and insults, but I mostly gave as good as I got there. And frankly, the kids giving me a hard time were having plenty of fun doing it; they were young, but they had plenty of agency of their own and they were getting their sadistic little jollies based on their own personal instincts. Some were mostly going along with the group, but it's not like they didn't have that side to them--they were accessing their worst side because that's what everyone was doing, and might have acted better if led a different direction, but that side was there for them to find.
I have little patience with the notion that it's all the parents' fault when kids act like vicious little creeps. That's one influence among many, probably smaller than peer group interactions, and personal character is a significant influence as well--kids are not tabula rasa even from very young, they bring their own personal stuff to the party.